1. Field of the Invention
The invention relates generally to exposure control apparatus for use in a camera, and more particularly to the use of a single electromagnetic device in an ambient light-regulated circuit to control a diaphragm and a shutter in accordance with the intensity of ambient light.
2. Description of the Prior Art
Exposure control apparatus in cameras often include a light-responsive electromagnetic circuit for controlling operation of a diaphragm and a shutter in relation to the intensity of ambient light. Typically, for example in U.S. Pat. No. 3,812,498, the circuit uses two electromagnets. One electromagnet controls adjustment of the diaphragm to reduce the effective size of an exposure aperture, and the other electromagnet controls closure of the shutter to terminate the exposure interval after the shutter has been opened. A light level measuring subcircuit comprising a photosensor and a series-connected resistor is often used for controlling the one electromagnet to adjust the diaphragm, and a light dependent R-C (resistor-capacitor) subcircuit comprising a photosensor and a series-connected capacitor is often used to control the other electromagnet to close the shutter.
It has also been proposed, for example in U.S. Pat. No. 3,385,187, to use a single electromagnet in conjunction with the light-responsive circuit to control both the diaphragm and the shutter. The single electromagnet is coupled with the light level measuring subcircuit and is initially energized to retain a locking pawl during the time a diaphragm, in the form of a Waterhouse stop, operates to vary the size of the exposure aperture. The electromagnet becomes de-energized to release the pawl to lock the diaphragm at a selected aperture size related to the intensity of ambient light. The action of locking the diaphragm also locks a shutter blade to prevent the blade from moving to re-cover the exposure aperture. The electromagnet is then coupled with the light dependent R-C subcircuit. At the end of an exposure interval as established by the R-C subcircuit, the electromagnet is re-energized and re-attracts the locking pawl to release the shutter blade, which then moves to re-cover the exposure aperture. Thus, the electromagnet becomes de-energized to set the diaphragm and subsequently is re-energized to initiate shutter closing. When the electromagnet is energized or re-energized to remove the locking pawl from its locking position, the magnetic force must act against the spring force holding the pawl in its locking position and must act against the mass of the locking pawl. The magnetic force needed for such operation is relatively high and requires a camera battery of corresponding high current capacity to reliably operate the electromagnet to attract the locking pawl. To reduce such high magnetic force and its attendant current drain on a camera battery, light spring forces are employed to hold the locking pawl in its locking position. However, light spring forces do not reliably lock the diaphragm or the shutter, and therefore the use of a single electromagnet in this manner to control both the diaphragm and the shutter does not provide effective use of the electromagnet.
An improved manner of using a single electromagnet to control both a diaphragm and a shutter is proposed in commonly assigned U.S. Pat. No. 4,059,836. In this instance, an armature couples the diaphragm, in the form of an aperture blade, and a closing blade of the shutter with the electromagnet. When a release member is initially depressed, the electromagnet is energized to hold the armature in the respective paths of movement of the aperture and closing blades. This prevents the aperture blade from moving to reduce the effective size of the exposure aperture after the aperture blade is freed by continued depression of the release member. Also, it prevents the closing blade from moving to re-cover the exposure aperture after the closing blade is freed by subsequent actuation of an opening blade to uncover the exposure aperture. If ambient light is relatively bright, e.g., above 200 footlamberts, the electromagnet is temporarily de-energized, which permits the aperture blade to move to reduce the effective size of the exposure aperture. Then, the electromagnet is re-energized, before the opening blade is actuated, to hold the closing blade removed from the exposure aperture. If ambient light is relatively dim, e.g., below 200 footlamberts, the electromagnet remains energized, i.e., is not temporarily de-energized, to hold the aperture and closing blades removed from the exposure aperture. After the opening blade has uncovered the exposure aperture for a duration related to the intensity of ambient light, the electromagnet is de-energized to permit the closing blade to re-cover the exposure aperture.
The use of a single electromagnet to control both a diaphragm and a shutter as proposed in U.S. Pat. Nos. 4,059,836 and 3,385,187, requires that the electromagnet be energized twice for each film exposure during relatively bright ambient light in the '836 patent and during any type of lighting condition in the '187 patent. Such multiple energization of the electromagnet for each film exposure (as compared to only one energization) is undesirable because of the attendant current drain on a camera battery, and therefore does not provide efficient use of a single electromagnet to control both the diaphragm and the shutter.